Cured silicone rubber gels (abbreviated as silicone gels, hereinafter) are used for electric or electronic potting or sealing, and as covering material for protection of control circuit elements, especially power transistor, IC, condenser and the like, against thermal and mechanical damages by taking advantage of their excellent electric insulation, stability to electricity and flexibility.
Various methods of preparing such silicone gels have been proposed. For instance, in the methods disclosed in Japanese Patent Kokoku (examined Japanese patent publications) Nos. 55-41705 and 55-38992 the silicone rubber is designed so that the ratio of hydrogen atoms attached directly to a silicon atom to vinyl groups attached directly to a silicon atom is one or less.
However, compositions which can be converted to such silicone gels by curing, and the silicone gels obtained suffer from a defect in that they are discolored through thermal deterioration.
Such a defect, though harmless in cases where electric insulation, electric characteristics, viscosity and the like are regarded as the significant properties because these properties of silicone gels undergo no change as a result of the discoloration, discoloration is a serious problem when optical characteristics are required of the silicone gels.
The discoloration of silicone gels which has accompanied thermal deterioration is known to be correlated to the amount of platinum used as the addition reaction catalyst, and the ratio of .tbd.SiH to .tbd.SiCH.dbd.CH.sub.2 groups. Specifically, the extent of discoloration becomes less the smaller the amount of platinum used, and the higher the .tbd.SiH/.tbd.SiCH.dbd.CH.sub.2 ratio. Therefore, the above-described discoloration resulting from thermal deterioration would seem to be preventable by reducing the amount of platinum used. However, a reduction in the amount of platinum used tends to cause curing troubles. When the factor of a high .tbd.SiH/.tbd.SiCH.dbd.CH.sub.2 ratio is added in particular, there is produced an additional a change in hardness and a foaming phenomenon are apt to occur as a result of thermal deterioration.
With the intention of bringing about an improvement in resistance to discoloration accompanying thermal deterioration, there has been proposed the combined use of polysiloxane containing at least two vinyl groups and one hydroxyl group attached directly to silicon atoms in the same molecule and a silane containing one epoxy group and one alkoxy group per one molecule. See examined Japanese Patent Kokoku No. 59-35932. In addition, Japanese Patent Kokai (unexamined published Japanese patent application) No. 62-39660 discloses a composition comprising an organopolysiloxane containing vinyl groups attached directly to silicon atoms which are numbered by 0.1 to 2.0 on the average per one molecule and a hydrogensiloxane represented by the formula, R.sup.4 Si(OSiR.sup.5.sub.2 H).sub.3, (wherein R.sup.4 represents an alkyl or phenyl group, and R.sup.5 represents an alkyl group). However, the silicone gels prepared from those compositions have the defect that the light transmittances at wavelengths shorter than 500 nm are lowered to 90% or below after they have suffered thermal deterioration, which limits the area of optical utilization.
As the result of concentrating on finding a solution to the problems as described above, it has now been found that the use of an organohydrogenpolysiloxane containing alkenyl groups and hydrogen atoms attached to silicon atoms at the molecular chain terminals is particularly effective, thus achieving this invention.